The method of measuring a substance to be measured (antigen or antibody) in a test sample, using a test reagent with particle suspension which suspends insoluble carrier particles carrying an antibody or antigen, utilizing the level of agglutination by the particles after reacting with the test sample as an indicator, is known as immunoagglutination and is widely used.
It has been reported that insoluble carrier carrying an antibody can be dispersed in a liquid phase with a low ionic strength having an NaCl concentration not higher than a prescribed concentration, or conversely, in a liquid phase with a high ionic strength having an NaCl concentration not lower than a prescribed concentration.
The insoluble carrier carrying a protein suspended in a solvent with a low ionic strength having a NaCl concentration not higher than a prescribed concentration, that is, suspended in a solvent having a low electric conductivity, is outstanding in effectively attaining good dispensability and sensitivity. However, non-specific agglutination may occur due to contamination of ions by impurities from biological samples.
It is known that interferences that influence the immunological reaction of the target substance exist in biological samples. To avoid the influence by the interferences of the immunological reaction, methods employing reagents with various additives have been widely used. For example, a method is known for using the reaction solution for detecting a target substance and the buffer solution, which is a second reagent containing an additive, for avoiding the influence from interferences. In many cases, the buffer solution has an electric conductivity (about 15 to 20 ms/cm) close to that of the saline. In measuring systems using an antibody suspended insoluble carrier under a condition of a low electric conductivity, non-specific agglutination may occur when the reaction solution is mixed with the buffer solution during the measurement.
In immunoagglutination or the like, various additives (for example, polyethylene glycol or guanidine) may be used in order to increase the sensitivity of the reagent. However, if an additive is used, the electric conductivity will increase, therefore non-specific agglutination may occur when the reaction solution is mixed with the buffer solution during the measurement.
On the other hand, in cases where an insoluble carrier carrying a protein suspended in a solvent having a high electric conductivity is used, self-agglutination does not occur in a short time, not within a several hours. However, self-agglutination takes place during long-term storage for several days or several months or more, so that the storage may not be attained.